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1-1-1984

Cambrian rocks of East Point, Nahant Massachusetts

Bailey, Richard H.

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Bailey, Richard H., "Cambrian rocks of East Point, Nahant Massachusetts" (1984). NEIGC Trips. 354.

https://scholars.unh.edu/neigc_trips/354

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Cl-1 249

CAMBRIAN ROCKS OF
EAST POINT, NAHANT,

MASSACHUSETTS

Richard H. Bailey
Department of Earth Sciences
Northeastern University
Boston, MA 02115

Introduction
The quintessential stratigraphic component of Avalonian terranes of eastern North America is a Cambrian succession bearing the so-called Acado-Baltic trilobite assemblage. Spectacular sea cliffs at East Point, the easternmost extremity of Nahant, Massachusetts, afford an opportunity to examine a continuous and well exposed Lower Cambrian section on the Boston Platform. The

Nahant Gabbro, sills, abundant dikes, and faults cut the Cambrian strata and add to the geological excitement. Indeed, it is difficult to move moret h a n afewmeters along the cliffs without discovering a feature that will arouse your curiosity. This is also a wonderful place to watch waves crash against cliffs and to

stare across the Atlantic in the direction of Africa.

Regional Lower Cambrian Stratigraphy
Cambrian rocks of Nahant are correlated with distinctive green and red nodular slates of the Weymouth Formation (LaForge,
1932) exposed in the Mill Cove area of Weymouth (Figure 1). Trilobites have never been found in Nahant Strata (a slate pebble with Strenuella was found; Grabau, 1900), but a thin zone in the

Weymouth Formation at the type locality (Burr, 1900; Grabau, 1900)

yielded a diagnostic t r i l o b i t e a s s e m b l a g e ( S t r e n u e l l a , C a l l a v i a , a n d W e y m o u t h i a ) i n d i c a t i v e o f t h e l a t e E a r l y C a m b r i a n o f t h e Acado-

Baltic faunal province and similar horizons in England and Morocco
(Shaw, 1950; Theokritoff, 1968; Anstey, 1979). The Hoppin
Formation at Hoppin Hill near North Attleboro, (Figure 1) also contains Strenuella strenua (Billings). A number of workers have

noted (Foerste, in Shaler and others, 1899; Grabau 1900; Theokritoff,
1968; Landing and Brett, 1982) that in both the Hoppin and Weymouth Formations the trilobite bearing assemblages overlie or are separated from a fauna dominated by small conoidal fossils. Landing and Brett
(1982) assigned the lower part of the Hoppin Formation, and the limestones of the Weymouth Formation at N a h a n t which lack trilobites, to the earliest Cambrian Tommotian Stage (Figure 1) and the

microfauna associated with t h e S t r e n u e l l a ( ( t r i l o b i t e ) bearing beds

at Hoppin Hill to the overlying Atdabanian Stage. The restriction of faunas to particular lithofacies, the very limited exposure

of good stratigraphic sections, the intimate association of the trilobite and non-trilobite assemblages, and the occurrence of a

trilobite(?) in limestone at Nahant suggest to me that the

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Cl-3 251

Lower Cambrian rocks of eastern Massachusetts are probably not Tommotian but lower Atdabanian or slightly younger. Theokritoff
(1968) explained the lack of co-occurrence of the trilobite and non-trilobite faunas as a probable result of facies control of assemblages and he therefore suggested that a non-trilobite

Coleoloides fauna need not necessarily be earliest Cambrian. Landing, Nowland, and Fletcher (1980) also report the extension

of several typical Tommotian phosphatic microfossil ranges well into the Callavia zone in Nova Scotia. We clearly need a better

understanding of the taxonomy, stratigraphic ranges, and paleoecology of many of the non-trilobite index fossils of the

earliest Cambrian. Until such knowledge is gained I suggest that application of the stage names of the Siberian Platform (Matthews and Missarzhevsky, 1975; Raaben, 1981) to the Cambrian successions

of isolated areas such as eastern Massachusetts may be somewhat premature.

Strata equivalent in tectonic setting, depositional environment, and fossil content to the Hoppin and Weymouth Formations are excellently exposed in eastern Newfoundland and in more limited exposures in Nova Scotia and New Brunswick

(McCartney, 1969). In Newfoundland and New Brunswick there is a profound, often angular, unconformity, with an overlying basal

quartzarenite, as at Hoppin Hill, followed by the Lower Cambrian succession (Skehan, 1969) . Kaye and Zartman (1980) have recently

proposed that the Cambridge Formation of the late Precambrian Boston Basin grades upward into the Weymouth Formation to form a continuous and conformable succession. The lithologies of the Weymouth and the Cambridge are quite distinct and different, especially when viewed on the outcrop scale. The well documented nonconformity at the base of the Cambrian at Hoppin Hill (Billings,
1929; Dowse, 1950), less than 40 km to the southwest of the Boston

Basin, is the result of transgression of shallowwater facies onto a stable continental(?) block. The shallow water facies of the Weymouth and Hoppin Formations, the uncomformable association with

stable basement, and the regional correlations do not support a basinal setting for the Weymouth Formation. Billings (1982)

argues, on structural grounds, that an uncomplicated transition between the Boston Bay Group and the Cambrian strata in Weymouth and Quincy is unlikely.

$

Nahant Stratigraphy
About 130 m of strata are exposed in northerly dipping beds of East Point. It is difficult to present a simple stratigraphic section as there are numerous fault offsets and 2 thick sills that interrupt the sequence. The section consists of a dark silicified mudstone or argillite with interbedded nodular horizons and limestone beds. A 3m thick limestone bed is found on the southeast side of the point along the top of the cliff. The three basic Cambrian lithologies are discussed below.

Cl-4 252

Argillite
Most of the section is a brittle, dark gray to black, thinly laminated argillite. Bedding is faintly visible in hand specimen.

Quartz silt and very fine quartz grains are present as thin, parallel laminae, and lenses (0.1-0.5 mm thick) or as scattered isolated grains. These laminae are separated by a dark

structureless mudstone. Some undulating contacts resemble scour surfaces. Very rare burrow-like structures are present.

Nodular Argillite
The most characteristic lithology of the Weymouth is a red, green, or black slate or argillite containing elongate carbonate nodules. The nodules are from 0.5 to 3 cm in thickness and most are about 3 to 15 cm in length. They occur with varying frequency

along bedding planes and occasionally are so abundant as to comprise a thin "limestone" bed. The nodules have been altered primarily by silicification, and by replacement with Ca-garnet, tremolite, wollastonite, and epidote. The highly altered nodules are often zoned or banded with chert and carbonate interiors and garnet and Ca-silicate rims (Bingham, 1977). Chertification initiated in the nodule often extended into the surrounding mudstone. I have seen no fossils in either the replaced or the relatively unaltered nodules. Underlying and overlying laminations

in the mudstone are often deflected and appear to be displaced by nodules suggesting that the mudstone was unconsolidated when the nodules formed. The shapes of nodules are reminiscent of algal

structures, but no definitive internal structure is present.

Limestone
White to light gray limestone beds range in thickness from
3 cm to 3 m. The very thin limestones usually occur in groups with interbedded, abundantly nodular, slate or argillite. Thicker limestones contain 0.5-3 cm thick, very thinly laminated, highly irregular, brownish or greenish chert beds. The cherts themselves

do not contain fossils; however, patches of fossiliferous chertified limestones are associated with the greenish layers. Fossils occur

in clearly defined thin zones within the limestones. These fossiliferous layers may be continuous and traceable for several

meters along the surface of the outcrops or they may be isolated as irregular masses.

Petrography of Limestones
The primary textures of limestones at East Point are remarkably well preserved given the proximity of igneous rocks. The rocks were originally biomicrites or fossil wackestones.

The micrite has generally recrystallized to very finely crystalline sparry calcite although some patches of microspar are present
(Figure 2). Abundant bioclast, primarily hyolithids, are composed

Cl-5 253

• >
• V
• M

5 mm
5 mm

Figure 2 Negative prints of acetate peels from slabs cut parallel to bedding. Microfacies labeled micrite (M), biomicrite (BM), biosparite or packstone (BS), scale bar is 2 cm. A.
(I) surrounded stone note oblique of Orthotheca searsi
(OS);
D. pocket hyolith packstone, surrounded by micrite. biomicrite and biosparite.; B. hyolith pack biomicrite mass surrounded by hyolith packstone;

Cl-6 254

A.
C.

Figure 3. Photomicrographs of thin taken with plane light. Scale bar is 1mm. A. Orthothecaprinceps

nested in Allatheca communis; B. nested
Circotheca

cylindrica; C. longitudinal of indeterminate hyolith; tallized hyolith cross and

; E. cross-isnteoc hyolith operculum(?)

whorl of Aldanella through about

h

______; G. longituthrough Stenotheca(?) cf. A. dinal

Cl-7 255

of blocky sparry calcite (Figure 3). This blocky spar may have filled voids created by the dissolution of original aragonitic shell material (James and Klappa, 1983). Although there are some

remaining void-lining, finely crystalline, prismatic crystals most have been recrystallized or obliterated by recrystallization of micrite matrix. Common stylolites can be identified in thin section by truncation of fossils.
Three microfacies are present: 1) very sparsely fossiliferous to unfossiliferous micrite occurring in thin beds, irregular patches, or as intraclasts (Figure 2); 2) sparsely to moderately

fossiliferous biomicrite or wackestone as thin beds or burrow (?) fillings that grade into unfossiliferous micrite or* 3) hyolithbiosparite, grainstone, or packstone. The latter microfacies is composed of abundant hyoliths and fragments. Some of these thin biosparite laminae overlie irregular scour(?) surfaces but more

commonly they are found as isolated pockets or nests of fossils surrounded by micrite intraclasts or biomicrite. The inter-

relationships of microfacies are illustrated in acetate peels of polished slabs cut parallel to bedding (Figure 2). Irregular masses of chert and epidote have secondarily altered areas of biomicrite and micrite.

Tubular and conoidal fossils do not show a strong preferred orientation. Long axis orientations (Figure 5) suggest only weak

current sorting. Nested, or cone in cone hyolith specimens are fairly abundant and, along with the considerable abundance of fossil fragments indicate moderate post-mortem movement by

currents. Long axes of many specimens of hyoliths are steeply inclined to bedding suggesting that the bottom may have been

somewhat hummocky or irregular.

Environment of Deposition
A shallow subtidal environment is inferred for limestone strata at Nahant. Evidence supporting this conclusion is:
1) irregular laminae of biosparite and biomicrite with

fragmental disarticulated shelly fossils
2) possible intraclasts around and between which accumulated biosparite and biomicrite
3) possible laminate and small domal or digitate stromatolites
(now chertified)
4) a relatively diverse (for Early Cambrian) calcareous macrofauna.
5) a general association of Lower Cambrian hyoliths with shallow water facies in areas where more complete stratigraphy allows a confident assessment of paleoenvironment. (especially Hoppin Hill and equivalent strata in Newfoundland) .
This environment was interrupted by short term fluctuations in energy level, possibly storms, to produce the bioclastic rich layers mixed with intraclasts. Micrite accumulated during

quiescent periods and during these periods stromatolites probably covered portions of the bottom. Limestone deposition ceased when

C l - 8 256

the influx of fine elastics, possibly associated with concomitant deepening of the platform halted biogenic carbonate production.

The thin nodular horizons result from a near balance between carbonate and clastic deposition. At no time during the deposition of the Nahant strata did a substantially uplifted extrabasinal

source exist. Quartz silt is the coarsest extrabasinal material present. A stable platformal setting is further indicated by basal quartzarenite over granitic basement at Hoppin Hill and by quartzarenites in proximity to, but not in actual depositional contact with, the Weymouth Formation east of Mill Cove (Billings,

1982) .

Paleontology
Foerste (1889) was the first to describe hyoliths from the
Cambrian strata of Nahant. Louis Agassiz in 1850 and Sears in
1887 had also noted the presence of fossils in Nahant limestones
(Grabau, 1900). By 1900 a well documented Lower Cambrian fauna was known from Hoppin Hill, Weymouth, Nahant, and from glacial

cobbles and boulders at several localities. These Early Cambrian fossils are of great interest because they include some of the first shelled organisms to appear on earth. My paleontological studies, and those of my students, utilize thin sections, acetate peels, acid etched blocks, and specimens obtained by dissolving

blocks. The discussion below lists and briefly describes taxa that have been found or reported from the limestone beds at Nahant.

Brachiopods
Grabau (1900) illustrated two species of inarticulate brachiopods, Obolella cf. 0. atlantica and Paterina bella. I have

found several internal molds and cross-sections indicative of the former species, but the material is of poor quality. Obolella

atlantica occurs with the trilobite Strenuella at both Hoppin Hill and Weymouth.

Hyoliths
The Nahant fauna is dominated by bilaterally symmetrical, conical fossils known as hyoliths. The taxonomic status of

hyoliths isuncertain, with some workers (Runnegar and Pojeta, 1974) regarding them as an extinct phylum, and some (Marek and Yochelson,
1976; Yochelson, 1978) placing them as an extinct class in the mollusca. A typical hyolith skeleton is a straight or slightly curved, rounded, flattened, or triangular cone closed by an

operculum. Two curved calcareous "whiskers"or appendages extending from the aperture (Yochelson, 1974) may have assisted in locomotion and or feeding (Figure 4). These creatures were probably benthic detritus feeders (Marek and Yochelson, 1976) . Much of the detailed systematic work on hyoliths, particularly those from the Lower

Cambrian, is based on specimens from the well exposed strata of the Siberian Platform (Raaben, 1981; Matthews and Missarzhevsky,
1975). The taxonomic assignment of sectioned hyoliths to genera

Cl-9 257

A.

Circotheca
A l l atheca
O r t h o t h e c a

p rinceps cvlindrica communis
I mm

Hy olithes (?) Tiksitheca (?) excellens americanus
Orthotheca searsi

......
/•;*

; • c.

mm

*.v*;

•%

H

' J : '*'**%*.

Figure 4 Sketches of Nahant fossils A. cross-

sections of hyolith genera; B. reconstruction of living hyolith (from Yochelson, 1974); C. Stenothecaabrupta
(after Sears, 1905, Fig. 209) D. camera lucida sketch of trilobite pygidium(P).

5 mm

Cl-10

and especially to species can be very difficult as one must constantly consider the geometric effect of obliquity of the section to the long axis of the specimen. Sections perpendicular to the long axis are most useful. The sections illustrated in

V

Figure 4 are diagnostic for Nahant species. The apical angle is also a variable but taxonomically useful character. Grabau (1900) listed 8 species of hyoliths from Nahant of which 6 are probably valid. Some species are illustrated in Figures 2 and 3. Identified species of hyoliths include: Circotheca cylindrica (Grabau) Allatheca communis (Billings)

Orthotheca searsi (Grabau) Orthotheca princeps (Billings)

Tiksitheca(?) americanus (Billings) Hyolithes(?) excellens (Billings)

Stenothecoids and Aldanellids
Stenothecoids are small, asymmetrical, inequivalved, bivalved organisms that possibly represent an extinct molluscan class. Specimens of Stenotheca abrupta (Shaler and Foerste) are present on acid-etched blocks and in thin section (Figures 3 and 4). Specimens have also been found at Hoppin Hill (Grabau, 1900).
Aldanella cf. A. attleborensis (Shaler and Foerste) was found in Nahant strata by Bingham C1977) and Landing and Brett

(1982). This tiny, loosely coiled trochisprial shell (illustrated in Figure 3) is considered by Runnegar and Pojeta (1974) to be the oldest known gastropod. Other workers (Yochelson, 1978; Landing and Brett, 1982) do not consider them to be gastropods, but possibly coiled worm tubes.

O

• 3 6 0
• 3 6 0

0.7 m N= 4 0
0.3 m N= 22
0.4 m N= 28

numbers give height above base of Is. bed and sample size non-conoidal fossils plotted in both opposite class intervals

Figure 5. Circular histograms (equal area) showing orientations of long axes of hyoliths measured in slabs and peels parallel to bedding.

Cl-11 259

Miscellaneous Specimens
Small straight or nearly straight tubes probably represent
Coleoloides sp., a possible annelid worm tube. Other enigmatic